Ferroelectric gates make transistors nonvolatile

PORTLAND, Ore.  Electronics devices using ferroelectric transistors would turn on instantly without the need to boot from flash or hard-disk memories. Such ferroelectric transistors would likely use a gate oxide with memory to create nonvolatile circuitry that retains its state when power is turned off. Many research efforts have pursued ferroelectric transistors, but so far all have failed for lack of a suitable manufacturing process.

A solution may be near: strained strontium titanate deposited on standard silicon substrates, according to a team of industry, university and government researchers. Engineers from Cornell University, the University of Pittsburgh, the National Institute of Standards and Technology, Penn State University, Northwestern University, Motorola Corp., Ames Laboratory and Intel Corp. participated in the ferroelectric research.

"Our work is an important step on the way to a ferroelectric transistor on silicon," claimed Cornell materials scientist Darrell Schlom. "We showed that strontium titanate could be deposited on silicon--strained by 1.7 percent in biaxial compression--and that it was indeed ferroelectric."

Ferroelectric gate oxides also would lower the energy required to turn a transistor on and off by virtue of a negative capacitance effect, thereby saving power and enhancing performance. Various nonvolatile topologies are possible using ferroelectric materials, such as those used by Ramtron International Corp., a supplier of nonvolatile ferroelectric random-access memory. But a ferroelectric transistor would make the state of all transistors in a device nonvolatile, not just its memory cells.

Texas Instruments, Sharp Laboratories of America, Infineon Technologies and others have all patented different approaches to harnessing ferroelectric materials in transistor structures that retain their states. Nevertheless, commercialization remains elusive.

Schlom acknowledged that the latest material breakthrough could still be hobbled by electronic traps at the interface or electrical leakage through the ferroelectric. Nevertheless, the Cornell-led team remains hopeful that it has found the key to manufacturing ferroelectric transistors by reducing the complexity of the materials required to fabricate its gate oxide.

"We have not made a ferroelectric transistor yet, but we have gotten rid of all of the intermediate layers," said Schlom.

Strontium titanate is a well known oxide, but it was not well known that straining it--compressing its atomic lattice to match that of silicon--could make it a good candidate for ferroelectric transistors. A related compound, strontium bismuth tantalate, is used in ferroelectric smart cards as is lead zirconium titanate. However, neither compound has proven to be good candidate as the gate dielectric for a transistor.

The research group claimed that strained deposition of strontium titanate on silicon is the answer. "By creating a ferroelectric directly on silicon, we are bringing the possibility of ferroelectric transistors closer to realization," said Schlom.

The theoretical prediction that strained deposition of strontium titanate on silicon could turn it into a useful ferroelectric gate oxide was made five years ago by Long-Qing Chen, a Penn State materials scientist who worked with Schlom on the recent proof-of-concept demonstration.

Chen found that strontium titanate was not ferroelectric in its relaxed state at any temperature. However, his mathematical models predicted that extremely thin films of the oxide, just a few monolayers thick, could become ferroelectric when its lattice was strained to match the spacing between the atoms of a silicon substrate.

"We knew exactly what we were after, but it took our team years to achieve and demonstrate the predicted effect," said Chen.

The researchers used molecular-beam epitaxy to deposit just a few atomic layers of strained strontium titanate on the silicon substrate. Jeremy Levy, a professor at the University of Pittsburgh, measured the results, thereby confirming that the atomically-thin layers of strained strontium titanate were indeed ferroelectric.

The researcher said they want to characterize the material further and eventually demonstrate working field-effect transistors using the strained strontium titanate as their gate oxide. The biggest remaining question is how long the ferroelectric effect can retain its memory. Previous attempts have resulted in memory retentions measured in milliseconds--as short as today's DRAMs--which would require constant refreshing of their states. But at least one research group, the National Institute of Advanced Industrial Science and Technology (Tsukuba, Japan), has reported ferroelectric gate transistors with memory retention of 10 days.

Funding for the research was provided by the National Science Foundation, the Office of Naval Research and the Energy Department.